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The mountain galaxias species complex is a group of freshwater Galaxiid fish found all over southeast Australia. They form a cryptic species complex.

These fish were originally designated as a single species, Galaxias olidus, despite:

• occupying a very wide geographic range;
• occupying a range of different habitats, from headwater rivulets at 1,800 metres on the flank of Australia's highest mountain (Mount Kosciuszko) to large "midland" rivers and streams (or in other words rivers and streams at the upland/lowland transition, which may be extensive in Australia);
• displaying a wide range of body forms and colouration.

Ongoing research is now revealing they are a species complex. In recognition of this, the mountain galaxias species complex has been referred to as Galaxias spp., although the designation Galaxias olidus will probably remain with one of the species in the complex. The mountain galaxias species complex also incorporates the barred galaxias (Galaxias fuscus) whose status as a distinct species was debated but is now more or less confirmed.

A specimen of the mountain galaxias species complex, caught in an upland stream on a tiny barbless wet fly. This population has since been destroyed by an illegal stocking of exotic trout.

The mountain galaxias species complex occupies a vast geographical range. They are found from southern Queensland to the Adelaide Hills in South Australia, and while occurring widely in the Murray-Darling river system, are also found in eastern and southern coastal systems as well. It is not clear how much of their coastal distribution is due to natural river capture events (although it is certain much of it is) and how much of it may be due to migration, for many mountain galaxias species have the ability to "climb" natural migration barriers with modified pelvic fin structures.

Within the Murray-Darling system the mountain galaxias species complex continues the trend of specialisation into upland and lowland habitats, with mountain galaxias species found in upland habitats, and the Flathead Galaxias found in lowland habitats. Though mountain galaxias species stray down to the upland/lowland transition zone in some river, mountain galaxias species are largely upland species. Indeed, they are the upland specialists, found in the smallest of streams and at higher altitudes in Australia than any other freshwater fish, as high in fact as streams go. There has been some fascinating micro-niche partitioning amongst the Galaxiids in upland Murray-Darling habitats, this being one of the causes of the species complex we have today.

The distribution of mountain galaxias species has been massively fragmented by the introduction of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss). Mountain galaxias have — with the exception of one remarkable newly discovered species that lives in water too fast for introduced trout — shown a complete inability to live with introduced trout species in upland habitats due to competition and predation, and extremely serious predation on mountain galaxias species by introduced trout has been documented. There have been countless localised extinctions of mountain galaxias species populations due to introduced trout, and this is continuing to occur with illegal trout stockings. These issues are widely recognised and documented in the scientific literature.

Many mountain galaxias populations, possibly undescribed species or subspecies, face extinction, and many other populations, also possibly undescribed species or subspecies, have been alreay been permanently lost.

• Galaxias olidus (TSN 623458). Integrated Taxonomic Information System. Accessed on 11 March 2006.
• "Galaxias olidus". FishBase. Ed. Ranier Froese and Daniel Pauly. 10 2005 version. N.p.: FishBase, 2005.
• McDowall, R.M. (2006) Crying wolf, crying foul, or crying shame: alien salmonids and a biodiversity crisis in the southern cool-temperate galaxioid fishes? Rev Fish Biol Fisheries 16: 233–422.

• Native Fish Australia - Mountain Galaxias page